Looking for a new lens for my d40

At the moment i'm leaning towards the Nikon AF-S VR 70-300mm

Nikon AF-S VR 70-300mm

But i've just found the following lenses

Tamron 70-300mm

and

Sigma 70-300mm

First off, i assume both those lenses will still be able to be used in auto focus mode with my d40?
Is the nikon lens worth the extra money?